236 A YEAR IN SCIENCE 



another into a stem tip, and others into one or more 

 leaves. After the embryo plant reaches this stage, in 

 most cases, the wall of the ovule becomes hardened and 

 the growth of the young plant is checked. This ovule 

 with the thickened wall and containing a plant in a 

 dormant condition is a seed. It may lie in this con- 

 dition for a long time and then if placed under 

 favorable conditions, the seed coat will burst and the 

 embryo plant will continue its growth into a young 

 plant. 



Pollination. Pollen is necessary for the production 

 of seeds. But before this pollen can fertilize an ovule, 

 it must first be transferred from the anther in which 

 it is formed to a stigma. This transfer of pollen from 

 anther to. stigma is called pollination. This process 

 appears to be one of the chief activities associated with 

 flowers. There are two kinds of pollination. Pollen 

 may be transferred from the anther to the stigma of 

 the sapie flower (self pollination), or to the stigmas of 

 other flowers (cross pollination). Charles Darwin, the 

 great English naturalist, found, about the middle of 

 the nineteenth century, that certain kinds of flowers 

 were entirely dependent for fertilization upon cross 

 pollination. He also found that some which were self 

 pollinated did not produce so many seeds, and that 

 plants which grew from those seeds were smaller and 

 weaker than plants from seeds produced by cross 

 pollinated flowers. He also found that another 

 advantage of cross pollination was that the plants so 



